This is an open-label study designed to assess the effectiveness of an oral thalidomide "swish and spit" solution on the healing of aphthous ulcers in patients with HIV infection. Patients will receive an oral thalidomide solution (containing 150 mg) four times daily for up to 4 weeks. Patients will be assessed weekly during the treatment period for clinical response and healing. At the end of the first 2 weeks, subjects noted to have an enlargement in ulcer size will be terminated from the study and referred to their primary provider for further care. Subjects who have had no reduction in lesion size at the end of 2 weeks will choose if they wish to continue to receive study drug for an additional 2 weeks or be terminated from the study and referred to their primary provider for further care. Data from the ACTG 251 study showed an approximately 60% complete response rate at the end of 4 weeks. Because of the known toxicities associated with systemic thalidomide use, the safety and potential efficacy of a topical solution of thalidomide is of interest. In this test of concept study, we will examine the response rate in 12 subjects in light of the response rate seen in the ACTG 251 study. Subjects will be assessed as to their degree of response at the end of the 4 week treatment period. Degree of response will be based upon clinical change in lesion length and width. The proportion of patients showing a l00% response at the end of the 4 week treatment period will be calculated. A sample size of 12 subjects was chosen. If no less then 9/12 subjects show complete healing, this would represent a proportion responding of (as low as) 51% or (as high as) 100%, based on confidence intervals. This response rate would be similar to that seen in the ACTG 251 study. The rate in the placebo group at the end of 4 weeks was 1/22 (5%). Even if the true rate of spontaneous healing (without treatment) is higher, the confidence intervals obtained with a response rate of 9/12 in this study would not overlap with a spontaneous response rate of as high as 50%. Eligible subjects include HIV-infected men or surgically sterile women at least 18 years of age with biopsy-confirmed aphthous ulceration of the mouth of at least 2 weeks duration by subject history and/or clinical observation. Biopsies will be obtained from the oropharynx of prospective enrollees. Those entering the study will have blood obtained on 2 occasions for thalidomide levels.